Supercapacitor is an energy storage device comprising two electrodes, a separator positioned between the two electrodes, an electrolyte and a package. Contrary to a battery that converts a chemical energy to an electrical energy through a redox reaction, the supercapacitor stores energy through surface absorption of charges. In this case, charge absorption takes place in the interface between an electrode and an electrolyte, and the supercapacitor undergoes very fast charge and discharge. An essential element of the supercapacitor is the electrode. The electrode is comprised of an electrode active material and a current collector on which the electrode active material is adhered. The current collector plays a role to conduct electrons. The electrode active material plays a role to absorb charges from the electrolyte. The electrode active material can be adhered to the current collector, for example, by a roller coating, a die molding and a dip coating.
The performance of the supercapacitor depends on a charge capacitance, a voltage and an internal resistance. The charge capacitance highly depends on a charge absorption rate. In order to enhance the charge capacitance, a carbon material having a high specific surface area and a large pore size, a metal oxide such as RuO2, IrO2, MnO2 or Co(OH)2, and a conductive polymer material is used as an electrode active material. Currently, activated carbon is widely used as an electrode active material. In order to accomplish high voltage in a system adopting the electrochemical supercapacitor, serial connection or stacking of unit cells of the electrochemical supercapacitor is adopted. Examples of the attempts to reduce the internal resistance include enhancement of binders (U.S. Pat. Nos. 4,327,400 and 5,150,283) and development of new electrode active materials (U.S. Pat. Nos. 5,079,674 and 6,512,667). Particularly, for the system such as electric vehicles that requires high energy in a short time, it is very important to minimize the internal resistance.
A metal thin plate used as a current collector is susceptible to oxidation. That is, an oxide layer is formed on the metal thin plate. Particularly, when an aluminum thin plate is used as a current collector, an alumina layer is formed on the surface of the plate. The alumina is an electrically nonconductive material, which increases the internal resistance of the supercapacitor.